A wide variety of laps are used in the polishing field to achieve smooth surfaces on a variety of substrates. When polishing, lapping or grinding, the surface of the lap is brought into contact with the surface of the substrate to be treated and relative movement is induced with respect to the substrate and the lap, resulting in smoothing of the substrate surface. A polishing media such as a particulate abrasive or an abrasive slurry is provided at the interface between the lap surface and the substrate to facilitate polishing. Typically, the polishing media is changed 2 to 4 times during the grinding and polishing procedure. Relative movement can be induced manually or mechanically. Examples of polishing laps are described by Wylde (U.S. Pat. No. 4,274,232), Takiyama et al. (U.S. Pat. No. 4,954,141), Duppstadt (U.S. Pat. No. 4,979,337), Smith (U.S. Pat. No. 4,980,995), Dillon (U.S. Pat. No. 5,095,660), Rotenberg et al. (U.S. Pat. No. 5,157,880), Pettibone (U.S. Pat. No. 5,205,083), Yu (U.S. Pat. No. 5,329,734) and Pasch (U.S. Pat. No. 5,403,228).
On a large scale, the lap surface has a shape corresponding to the desired general shape of the substrate to be treated. For example, the lap surface can be, in general, flat or, if the substrate is to have a concave surface, then the lap will have a corresponding convex surface. On a smaller scale, the lap surface is textured. Texturing facilitates dispersion of the polishing media over the lap surface as well as provides areas that can act as reservoirs for the polishing medium and for the material removed from the surface of the substrate being treated. The lap surface can be further provided with embedded abrasive particles to facilitate polishing.
The lap itself is often made of a hard material, such as cast iron or ceramic. These materials are accurately machined to achieve the desired overall shape, as well as the smaller scale texturing of the lap surface. During the polishing, lapping or grinding, the surface of the lap must be monitored to determine whether any changes occur. Changes in the surface conditions of the lap can induce imperfections in the substrate surface being, e.g., polished. As a result, if such changes occur, the lap must be replaced. Also, if the lap surface contains embedded abrasive particles, the abrasive size can change during the process, thereby requiring tedious lap cleaning and reconditioning procedures. Although good surface accuracy can generally be obtained using such hard laps, one is not often able to achieve the best surface finish. A further disadvantage associated with laps made of metal materials, such as cast iron, is that such laps may not possess adequate chemical resistance to the polishing media being employed which can be highly acidic, highly alkaline or in other ways reactive with the metal itself.
To eliminate some of the above-mentioned problems, it is known to use polishing cloths in conjunction with hard laps. In such cases, the hard laps are shaped as required to provide both the desired overall shape and texturing and then the lap surface is faced with a pad made of various materials, such as felt, velveteen or synthetic fabrics. While such laps can provide good surface finish when used with appropriate polishing media, there is a disadvantageous loss of dimensional control with respect to the shape of the substrate being treated.
Pitch laps having desired overall shape and local texture are also used in the polishing field. However, the production of such laps is a tedious process involving first melting the pitch and then pouring it onto an appropriate support substrate. Thereafter, once the pitch has cooled and hardened, the surface is cut to give a desired pattern, such as cross-hatching. The pitch lap is then pressed against a master surface or the substrate to be treated to generate the desired contour of the lap. While it is possible for such pitch laps to initially provide the desired finish and contour accuracy, maintaining the overall shape of the pitch lap can be difficult since pitch is a viscous fluid which continues to exhibit some degree of flow even at room temperature. A further disadvantage is that, if a pitch lap becomes contaminated by, for example, foreign particulate matter, or if its surface becomes damaged, a new lap must be made from scratch.
Chemically resistant polymeric laps are also known in the polishing art. In these laps, a polymeric material such as polytetrafluoroethylene (PTFE) is applied to a hard substrate and then treated to achieve the desired shape and texture. The elasticity of the polymeric layer can provide for the achievement of a good surface finish. However, the manufacturing of such laps is a time consuming procedure. Further, contamination of the polymeric layer by even a single large hard particle can render the lap useless and, thus, a new lap must be made again.